1. Field of the Invention
The present invention relates to a dual fuel nozzle which is capable of injecting either a gaseous fuel or a liquid fuel into the combustion chamber of, for example, a gas turbine.
2. Description of the Related Art
An engine operating on either a gaseous fuel or a liquid fuel, as required, such as a gas turbine, is equipped with dual fuel nozzles capable of supplying either a gaseous fuel or a liquid fuel to the combustion chamber (combustor) of the engine. Usually, a dual fuel nozzle is provided with separate injection holes exclusively used for a gaseous fuel and a liquid fuel. Further, a dual fuel nozzle is provided with atomizing holes used for injecting atomizing steam or water when liquid fuel is used. Atomizing steam or water is used for atomizing the liquid fuel, and thereby supplying liquid fuel to the combustion chamber in the form of very fine particle in order to suppress exhaust smoke.
FIG. 3 shows a typical longitudinal section of a conventional dual fuel nozzle of a gas turbine and FIG. 4 is an end view of the nozzle viewing from the direction indicated by the line IVxe2x80x94IV in FIG. 3.
In FIG. 3, reference numeral 3 designates a dual fuel nozzle as a whole, 1 designates an inner tube of the combustor of a gas turbine. The dual fuel nozzle 3 is provided with a nozzle tip 6 at the end thereof. A liquid fuel injection hole (a tip hole) 9 for injecting liquid fuel is disposed at the center of the nozzle tip 9 and, as shown in FIGS. 3 and 4, atomizing holes 10 and gaseous fuel injection holes 7 are disposed concentrically around the nozzle tip 6. Further, swirlers 2 for forming a swirl of combustion air are disposed between the nozzle 3 and the inner tube 1.
Combustion air is supplied through an air passage 2a formed by an annular space between the nozzle 3 and the inner tube 1. Combustion air in the air passage 2a forms a swirl when it passes through the swirler 2 and flows into the combustion chamber (the inside of the inner tube 1).
When gaseous fuel is used, fuel is supplied to a gaseous fuel passages 7a and injected into the inner tube 1 from the gaseous fuel injection holes 7. Gaseous fuel injected from the gaseous fuel injection holes 7 burns in the combustion chamber and forms a diffusion flame 8, as shown in FIG. 4. On the other hand, when liquid fuel is used, liquid fuel is supplied to a liquid fuel passage 6a and injected from the liquid fuel injection hole 9 of the nozzle tip 6 into the swirl of combustion air and forms the diffusion flame 8. Further, when liquid fuel is used, steam or water is injected from the atomizing holes 10 in order to atomize the liquid fuel injected from the liquid fuel injection hole 9.
However, in the conventional type dual fuel nozzle in FIGS. 3 and 4, especially when the amount of fuel injection is small, vibratory combustion may occur. An engine such as a gas turbine is required to operate over a wide load range. Thus, the amount of fuel injected from the nozzle changes widely in accordance with the change in the engine load. Therefore, in the conventional dual fuel nozzle, the injection holes must have large diameters so that a sufficient amount of fuel can be injected there through when the engine load is high. However, if the injection holes having large diameters are used, it is necessary to reduce the fuel supply pressure largely in order to reduce the fuel injection amount when the engine load is low. When the fuel supply pressure becomes low, the difference between the combustion chamber and the fuel supply pressure (i.e., the pressure difference across the fuel nozzle) becomes small. When the pressure difference across the fuel nozzle is low, the amount of fuel passing through the nozzle, i.e., the fuel injection amount changes largely in response to fluctuation of the pressure in the combustion chamber. Further, the change in the fuel injection amount causes changes in the combustion pressure (the pressure in the combustion chamber). Therefore, the fluctuation of the pressure in the combustion chamber is amplified and vibratory combustion occurs if the frequency of the fluctuation of the pressure in the combustion chamber matches the hydrodynamic natural frequency of the fuel supply system. This causes unstable combustion in the combustion chamber and a low frequency combustion vibration in which vibration and noise due to cyclic change in the pressure in the combustion chamber occur. The combustion vibration occurs when either gaseous fuel or liquid fuel is used if the pressure difference across the fuel nozzle becomes low.
Therefore, in the conventional dual fuel nozzle, it is necessary to keep the fuel injection amount at a relatively large value in order to suppress combustion vibration. This cause a problem when the conventional type dual fuel nozzle is used as a pilot burner for a premixed combustion type low NOx combustor. The premixed combustion type low NOx combustor is a combustor which reduces the amount of NOx generated by combustion by lowering the combustion temperature by burning fuel in a premixed combustion mode in the combustor. However, if the conventional dual fuel nozzle is used for a pilot burner, since the fuel injection amount must be kept at a relatively large value in order to suppress combustion vibration, it is difficult to lower a pilot fuel ratio (a ratio of the fuel injection amount of a pilot burner to a total fuel injection amount of the combustor). In this case, since the fuel injected from the pilot burner burns in a diffusion combustion mode as explained before, a relatively large amount of NOx is produced by the pilot burner due to a relatively high temperature of the diffusion combustion. Therefore, the amount of NOx produced by the premixed combustion type combustor increases as the pilot fuel ratio becomes larger. Consequently, if the conventional dual fuel nozzle is used as a pilot burner for the premixed combustion low NOx combustor, it is difficult to reduce the amount of NOx sufficiently.
Further, since the conventional dual fuel nozzle requires atomizing holes for injecting steam or water in addition to the gaseous fuel injection holes and liquid fuel injection holes, the construction of the nozzle is complicated.
In view of the problems in the related art as set forth above, the object of the present invention is to provide a dual fuel nozzle having a simple construction and being capable of suppressing the combustion vibration when the fuel injection amount is low.
The object as set forth above is achieved by a dual fuel nozzle for injecting gaseous fuel and/or liquid fuel into a combustion chamber, according to the present invention. The dual fuel nozzle is provided with a first injection hole and a second injection hole for injecting fuel therefrom, wherein the second injection hole has a diameter smaller than the first injection hole and, when gaseous fuel is used, the nozzle injects gaseous fuel from one of the first and the second injection hole, or both injection holes depending upon the required amount of fuel injection and, when liquid fuel is used, the nozzle injects a mixture of liquid fuel and steam from the second injection hole.
According to the present invention, the dual fuel nozzle is provided with the first injection hole and the second injection hole having a diameter smaller than the first injection hole. When gaseous fuel is used, fuel is injected from the first injection hole or the second injection hole, or both injection holes depending on the amount of fuel injection. For example, when the fuel injection amount is large, gaseous fuel is injected from both of the first and second injection holes. Therefore, a large amount of fuel can be injected into the combustion chamber. When the fuel injection amount is medium, gaseous fuel is injected only from the first injection hole having a larger diameter. When the fuel injection amount is small, gaseous fuel is injected only from the second injection hole having a smaller diameter. Since the second injection hole has a smaller diameter, the flow resistance thereof is high. Therefore, by using the second injection holes, the pressure difference across the nozzle remains large even when the fuel injection amount is small. Consequently, when gaseous fuel is used, the sensitivity of the fuel injection amount to the fluctuation of the pressure in the combustion chamber becomes low, and combustion vibration in the low fuel injection amount operation is effectively suppressed.
Further, when liquid fuel is used, liquid fuel is premixed with steam before it is injected into the combustion chamber. This mixture of fuel and steam is injected from the second injection hole having a smaller diameter. Therefore, the velocity of the mixture passing through the nozzle is kept high even when the fuel injection amount becomes low. This maintains the pressure difference across the nozzle sufficiently high to suppress the combustion vibration when the fuel injection amount is small. Further, since the velocity of the mixture of liquid fuel and steam injected from the second injection hole is high, good atomization of the liquid fuel is obtained without using separate injection of atomizing steam or water. Thus, the dual fuel nozzle of the present invention does not require separate atomizing holes for injecting atomizing steam or water, and thereby the construction of the nozzle becomes largely simplified.
The dual fuel nozzle according to the present invention may be used as a pilot burner or a main burner of a gas turbine combustor. If the dual fuel nozzle according to the present invention is used as a pilot burner for a premixed combustion type low NOx gas turbine combustor, the pilot fuel ratio can be largely reduced and, thereby, the total amount of NOx produced by the combustor can be sufficiently reduced.